1. Field of the Invention
The present invention relates to a high density array of detectors for an X-ray scanning apparatus and the like.
2. Description of the Prior Art
Present computed tomography (CT) scanners and digital radiography systems used several hundred to several thousand X-ray detectors. Each X-ray detector includes a scintillator to convert X-rays into light and a photocell to convert that light into an electrical signal. In CT scanners as well as in digital radiography systems, it is important that the detectors have equal pitch, that the center-to-center distances from detector to detector are equal. It is also important that the detectors have maximum density. That is, the detectors need to be located as close as possible to one another thereby resulting in a detection system which has a high detection efficiency so that a patient need be exposed to only the minimum amount of X-ray to produce a satisfactory image.
Today, CT scanners and the like may conventionally employ either discrete X-ray detectors or array X-ray detectors.
In FIG. 1 there is illustrated a plurality of discrete X-ray detectors. Each such X-ray detector includes a photocell 10 which is mounted on an individual substrate 12. A scintillator 20 is provided for each photocell 10 to convert X-ray radiation into light to which photocells 10 are sensitive.
As is well known to those skilled in the art, each individual photocell 10 must be positioned at least a distance 14 from each edge 16, 18 of substrate 12 to permit sufficient cutting and kerf allowances to interrupt substrate 12 at each edge 16, 18 without damage to photocells 10. If photocells 10 were closer to edges 16, 18 of substrate 12 than distance 14, substrate 12 could not be diced, cut or otherwise interrupted to form edges 16 and 18 without damage to photocells 10.
Each photocell 10 is illustrated in FIG. 1 as being of equal width W.sub.1. Each photocell 10 is separated from an adjacent photocell 10 by a distance L.sub.1 equal to twice the distance 14. With the arrangement illustrated in FIG. 1, the center of each photocell 10 is separated from the center of another by a pitch P.sub.1 equal to W.sub.1 plus L.sub.1.
Accordingly, when using discrete detectors as illustrated in FIG. 1, it is quite easy to achieve equal pitch for all detectors. However, when using a discrete detector scheme as illustrated in FIG. 1, a significant amount of space L.sub.1 is required to be left between individual photocells 10. In addition, use of discrete detectors requires use of discrete scintillators 20 which are spaced apart from each other a finite distance. This leads to two problems. First, such an arrangement reduces the available signal and, hence, results in a poor signal-to-noise ratio. In addition, such an arrangement permits many X-rays to
fall on dead areas between scintillators and, hence, such X-rays are not detected. This results in an increased required patient dose for a given amount of information gathered.
Separation L.sub.1 between photocells can be reduced using an array detector arrangement as illustrated in FIG. 2. As was the case with the discrete detector scheme of FIG. 1, each photocell 22 is provided with a corresponding scintillator element 26. In FIG. 2, however, a large number of photocells 22 are located on a single substrate 24. Accordingly, photocells 22 may be positioned adjacent one another a distance L.sub.2 apart, which distance L.sub.2 is limited only by the masking and production techniques utilized to produce photocells 22. Accordingly, L.sub.2 may be substantially smaller than L.sub.1 of FIG. 1, thereby increasing available signal strength, reducing signal-to-noise ratio and also reducing the amount of dead area upon which X-rays may fall. In such an arrangement, moreover, there would be an equal pitch W.sub.2, where W.sub.2 equals W.sub.1 plus L.sub.2.
The density of photocells 22 may be maximized by making the entire array of thousands of detectors on a single substrate. However, if one of photocells 22 were to malfunction, the entire array would be worthless, since L.sub.2 is too small to permit cutting, dicing or otherwise interrupting substrate 24 between adjacent photocells 22. Also, presently, it is not possible to fabricate an array larger than 4-5 inches in length because of the size limitation of available semiconductor wafers.
It would be possible to limit the number of photocells 22 on a single substrate, thereby minimizing the negative effects of having a single bad photocell within a group of detectors on a single substrate. However, each photocell 22a positioned adjacent an edge 28 of substrate 24 would need to be located a distance L.sub.1 from that edge thereby resulting in a pitch P.sub.1 between adjacent photocells 22a, in contrast to the standard pitch P.sub.2 between adjacent photocells 22. Moreover, if equal width scintillator elements 26 are employed, a resulting gap G would be experienced between scintillator elements 24a which are associated with photocells 22a. This gap G introduces a new dead area and, hence, results in an increase in the required patient dosage for a given amount of information to be gathered.
It is, accordingly, an object of the present invention to provide a high density array which exhibits a uniform pitch between detectors and which permits selective removal of a limited group of detectors should any photocell of a detector within that group prove faulty.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.